We obtained ratings on the Conners' scales from teachers (CTRS-28) and parents (CPRS-48) for 61 monozygotic and 64 dizygotic twin pairs, aged between 7 and 11 years. Model-fitting analyses were carried out to estimate the extent of genetic and environmental influences on problem behaviours, and to explore possible contrast effects in ratings by parents and teachers. Confirming previous findings with other measures, there was evidence of moderate to strong genetic effects on a range of problem behaviours. Parents' ratings on the Anxiety, Impulsive-Hyperactive and Learning Problem sub-scales showed significant evidence of contrast effects. There was no evidence of such rater bias or competitive sibling interaction effects in ratings by teachers, or in parents' ratings on the Conduct Problem and Psychosomatic sub-scales. Twin Research (2000) 3, 251–258.